Rubber compositions are typically used for tire treads which may be optimized for various rubber composition properties to promote one or more of three tire properties; namely, traction, rolling resistance and treadwear.
In this regard, typically desirable physical properties for tire tread rubber compositions may include, for example, hysteresis, hardness and modulus. Some properties are normally indicated by their rebound properties, tangent delta (Tan. delta) at 0.degree. C., and abrasion resistance. Hysteresis is conventionally related to Hot Rebound values. Such physical properties are well known to those having skill in the rubber compounding art and, in general, are considered to be somewhat predictive of tire tread performance.
More specifically, it is sometimes desired that a tire have a relatively low rolling resistance to enhance a vehicle's fuel economy. A lower hysteresis, usually evidenced by a higher hot rebound value, of a cured, or vulcanized, rubber composition is predictive of reduced heat build-up for the rubber composition and, therefore, of its utility for a relatively low rolling rubber composition for use as a tire tread.
It is often an accepted practice to reduce particulate reinforcement of a rubber composition to a relatively low level to reduce its hysteresis and thus, a predictive reduction in rolling resistance for a tire tread application. For example, where it might be desired to reduce a tire's rolling resistance by reducing hysteresis increasing hot rebound values of a rubber composition for a tire tread application, particulate reinforcement such as carbon black and/or silica might conventionally be reduced from a somewhat normal range of about 55 to about 80 phr to a reduced amount of such reinforcement in a range of about 30 to about 50, and particularly about 30 to about 45, phr. Such reduction in particulate reinforcement for a tire tread rubber for reduction in a tire's rolling resistance is well known to those having skill in such art.
However, commensurate with rolling resistance of the tire tread composition reduction by reducing reinforcing filler content, with other aspects of the rubber composition being essentially unchanged, the tire tread wear resistance often decreases, as may be evidenced by an increase in the tire's treadwear, and there is usually a degree of loss in tire tread traction. In one aspect, a tire's increase in treadwear can sometimes be predicted or somewhat correlated, reduction in the cured rubber composition's abrasion resistance.
Accordingly, it is desired herein to provide a tire tread rubber composition with reduced hysteresis (increased hot rebound values) while substantially maintaining an acceptable abrasion resistance, and usually an acceptable tire tread traction.
It is desired to provide such rubber composition with low levels of carbon black and/or silica reinforcement.
It is recognized herein that calcium carbonate has sometimes been used as a relatively low cost filler and extender for various polyolefin resins and some elastomers. However, it is believed herein that it has not been used in tire treads with a relatively low carbon black content together with specified modifier(s).
In the description of this invention, the term "phr," where used herein, and according to conventional practice, refers to "parts of a respective material per 100 parts by weight of rubber or elastomer".
In the description of this invention, the terms "rubber" and "elastomer," if used herein, may be used interchangeably, unless otherwise prescribed. The terms "rubber composition," "compounded rubber" and "rubber compound," if used herein, are used interchangeably to refer to "rubber which has been blended or mixed with various ingredients and materials" and such terms are well known to those having skill in the rubber mixing or rubber compounding art.